1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device provided with a semiconductor element that is composed of a semiconductor film formed over a transparent substrate, and particularly to a technique that is effective to be applied to a method of manufacturing a semiconductor device having a crystalline semiconductor film, typically, a silicon film.
2. Description of the Related Art
In recent years, there has been a progress in a development of a semiconductor device in which a thin film transistor (TFT) is formed with a semiconductor thin film (having a thickness in the range substantially from several nanometers to several hundred nanometers) formed over a substrate having an insulating surface and a large area integrated circuit is formed with the TFT.
As a related method of fabricating a TFT, there is a method in which a metal element is added to an amorphous semiconductor film followed by forming a crystalline semiconductor film at a low temperature (600° C. or less) and for a short time (from 1 hour to 12 hours) (hereinafter referred to as “solid phase epitaxy with metal element”) (Reference 1: Japanese Patent Laid-Open No. 7-130652).
Further, there is also a method of fabricating a semiconductor film in which after the solid phase epitaxy with metal element, a crystalline semiconductor film is formed by laser irradiation (hereinafter referred to as “laser annealing”) (Reference 2: Japanese Patent Laid-Open No. 7-161634). According to the technique of the Reference 2, the crystallinity of a semiconductor film is improved, resulting in an improvement in the electrical characteristics of a thin film transistor having the semiconductor film.
It is known that the crystalline semiconductor films fabricated according to the Reference 1 and Reference 2 have different concentrations of metal elements.
FIG. 6 shows concentration ratios of nickel to silicon in crystalline silicon films after the removal of the Ni element measured with TXRF (Total Reflection X-ray Fluorescence Spectroscopy). The result of measurements of sample A and sample B are shown in FIG. 6, wherein sample A indicates a crystalline silicon film fabricated according to the technique of the Reference 1 (solid phase epitaxy with metal element), and sample B indicates a crystalline silicon film fabricated according to the technique of the Reference 2 (solid phase epitaxy with metal element and laser annealing). As shown in FIG. 6, the crystalline silicon film fabricated according to the technique of the Reference 2 (sample B) is lower in the concentrations of Ni in the film, that is, in the concentration of a metal element. Accordingly, by processing under the condition of a lower temperature for a shorter time according to the technique, fabricating a TFT that is lowered in off-current and power consumption can be realized.
On the other hand, the inventors have found that the crystalline semiconductor film fabricated by laser irradiation after heating an amorphous semiconductor film added with a metal element according to the Reference 2 has a distortion resulting from crystal defects in a vicinity of a substrate or an insulating film for blocking impurities from the substrate.
FIG. 5A is a TEM diagram of a section of a crystalline semiconductor film obtained by exposure to an excimer laser beam. Meanwhile, FIG. 5B is a schematic diagram of the FIG. 5A. FIGS. 5A and 5B show: a region *1 denoting a crystalline semiconductor film; a region *3 denoting a silicon nitride oxide film as a blocking film; a topside of the region *1 (region *4 in FIG. 5B) denoting an amorphous silicon film as a protective film; and a region *2 and a region *2a respectively denoting defect parts of the crystalline silicon film.
It is considered that such defects are developed since a semiconductor film, typically a silicon film, is not completely melted by the laser annealing with an excimer laser beam. In the case where a semiconductor film having such defect parts is used in a TFT, the electrical characteristics of the TFT is adversely affected. A potential level in a grain boundary caused by the crystal defects and the like causes a deterioration of the carrier mobility. Accordingly, in the TFT fabricated according to the technique of the Reference 2, it is considered that, when the crystal defects in the semiconductor film can be reduced, on-current and mobility can be improved and an S value can be reduced, resulting in a further improvement in the electrical characteristics of the TFT.
Furthermore, it is known that the solid solubility of a metal element is higher in an amorphous portion than in a crystalline portion. It is inferred that since a crystal defect part is lower in the crystallinity, the metal element tends to segregate in the crystal defect part. From these, it is considered that when the crystal defects in the semiconductor film are reduced, and a concentration of residual metal element segregating in the defect parts is reduced thereby, similarly, the electrical characteristics of the TFT can be improved.